Mineral oils such as typical lubricating oils in which the main constituents are hydrocarbons are liquids which have good insulation. It has long been known that when such liquids are conveyed through pipes or the like, static electricity (which can be referred to as an electrostatic flow) occurs, as reported in Aichi Industrial College Research Reports B Specialist Discussion Papers 14, 1-6, “Electrostatic flows of liquids in narrow pipes” 31 Mar. 1979).
The electrical charge then created is carried to storage tanks together with the liquid, and there are cases where sparks occur inside the tanks or in the vicinity thereof because of the electrostatic charge or release of electricity and the liquid ignites. Known ways to suppress the accumulation of static electricity and to prevent sparking in relation to this phenomenon include, for example, adding Stadis-450 (made by the DuPont company) in which dinonylnaphthylsulphonic acid is the active constituent to increase electrical conductivity (or conductance or specific conductivity).
Furthermore, in recent years the risk of occurrence of static electricity has increased because of seeking higher performance from hydraulic apparatus and the higher speeds at which oils are conveyed. Sparks occurring because of electrostatic phenomena on the surfaces of solids or oils in such storage tanks manifest themselves as “noise”, and give rise to problems in that they cause misactuation or failures in control apparatus which contains electronic parts.
In particular, hydraulic oils are liquids that transmit a motive force and are used in functions such as transmitting a motive drive, controlling power or acting as dampers in hydraulic systems such as hydraulic actuators and other apparatus. They also perform the function of lubricating sliding or rubbing parts.
The most recent hydraulic devices have kept on becoming more compact and delivering higher outputs. At the same time, where actuation pressures were once of the order of, say, 14 to 20 MPa, they have now gradually increased to over 30 MPa, and the rate at which the oil is conveyed has increased, so that the likelihood of an electrostatic flow occurring is even higher.
These hydraulic systems are normally fitted with electronically controlled valve systems, which means that not only are they designed to eliminate sparking “noise” but also that it is desirable that they have oils which, from the safety aspects during storage, have high flash points. In addition, there is demand for solutions that have suitable friction coefficients so that there are no control issues in the case of lubrication of wet brakes.
It is known that, in order to improve the conductivity of lubricating oil compositions, it is possible to incorporate in the base oils additives having in their molecules strong polar groups such as organometallic compounds, succinic acid derivatives or amine derivatives, and lipophilic groups of suitable size in combination with aromatic azo compounds. The qualifying standard for these is a volume resistivity of not more than 1×1010 Ω·cm. This is of an order corresponding to at least 10 pS/m as a siemens (symbol S) rating and has still been insufficient to prevent reliably the occurrence of sparks due to electrostatic flow. Also, because aromatic azo compounds are a necessary ingredient, they give the lubricating oils a red colour and it has become difficult to evaluate the ageing of lubricating oils by means of visual inspection on site, so that there is then concern over braking or damping characteristics as indicated in Japanese Laid-open Patent 2001-234187.
This invention is intended to offer a lubricating oil composition for use in hydraulic actuators which imparts electrical conductivity in order to inhibit the occurrence of the “noise” which has a detrimental effect on electronic control mechanisms such as those that electronically control valve systems, and which also has superior braking properties for wet brakes controlled electronically, and which further has superior safety.